The present invention relates to a room-security system wherein one or more transmitting elements and one or more receiving elements are relied upon for detection of intruder presence or motion within the security area.
Known room-security systems are based on different physical techniques and principles. Thus, it is known from West German Pat. No. 2,653,111 to detect the presence of an object, for example, an unauthorized person or a burglar, within a secured space or area by the infrared radiation (body radiation) coming from said object; such detection utilizes suitable detectors and reflector surfaces which reflect, onto a common radiation receiver, radiation impinging from different separate receiving areas.
This method has the advantage of a very low average current consumption; however, systems utilizing this method are disadvantageous in that they are characterized by a relatively poor security factor and by too-frequent false alarms which, in principle, cannot be eliminated, even by the use of plural reflector surfaces which focus the radiation impinging from different separate receiving areas onto a common radiation receiver. These disadvantages result from the involved basic physical principle and from the nature of electromagnetic radiation within the region between 10 and 30 .mu.m.
Furthermore, from West German Pat. No. 3,032,510 a room-security system is known which employs a source of light as an energy radiator, and an optoelectronic receiver as the energy receiver; the said source of light radiates ultraviolet light, and energy impinging on the receiver is varied by persons or objects entering into or moving within the space to be secured and monitored. This system is provided with an evaluation circuit and is characterized by the fact that the light source diffusely distributes UV radiation within the room and that the optoelectronic receiver is shielded from the source of light; the UV radiation is radiated with pulse modulation, and an electronic receiving device receives the modulated UV radiation and, recognizing danger (or a disturbance in operation) from a disturbance of the pulse code, forwards a signal to give off the alarm.
This security method, however, has the disadvantage, in addition to the health-impairing effects of UV radiation, that it results in unavoidable, relatively high current consumption. Since the source of light distributes UV radiation diffusely in the room, a person who approaches the secured region from the outside produces a continuous change in the signal at the receiver. The reliable detection of an alarm condition, as well as the differentiation between alarm and disturbance, is therefore not realizable with sufficient reliability, so that completely reliable use of this room-protection system is not assured, and false alarms are again too frequent.
Other intruder-detection methods are based on ultrasonics and utilize the Doppler effect. The Doppler signal which is formed is, however, dependent on the relative speed of the entering person relative to the surrounding air and the nature of air circulation in the room; and the range of speeds can change greatly as a function of limiting conditions, such as temperature and the condition of the windows and doors, whether closed or open. For these and other reasons, these Doppler systems are extremely susceptible to failure.